The present invention generally relates to solid state extrusion of semi-crystalline fluoro-polymer films, more specifically, to solid state extrusion of ferroelectric polymer films below the Curie transition temperature of the ferroelectric polymer.
Many polymers, copolymers, and polymer compositions can be fabricated into polymer films having a thickness of about 1000 micrometers or less. In general, polymer film forming methods include solution casting, melt pressing, and melt extrusion. Solution casting involves pouring (casting) a solution of a polymer in a solvent onto a surface and evaporating the solvent leaving a film on the surface. Melt pressing and melt extrusion of thermoplastic polymers may include forming films by pressing a molten polymer between plates or extruding the molten polymer though a slit, followed by cooling below the melting temperature of the polymer. Films with molecular orientation can be formed from semi-crystalline polymer films by biaxial tensile drawing (i.e., stretching) the film utilizing methods known in the art. Polymer films may also be formed using compressional techniques, which may include deformation of a polymer into a film by forging or extrusion of the polymer at or near the polymer melting point (e.g., solid state deformation or extrusion).
Various semi-crystalline organic polymers may have crystallites that may exist in more than one crystal morphology. These crystallites may be interconvertible while in the solid state. Typically, such polymers may exhibit a solid-solid transition (that is, a crystal-crystal transition) involving the inter-conversion of the crystal forms at a particular transition temperature. Examples of such polymers include those referred to as ferroelectric polymers which often exhibit a crystal-crystal transition from a ferroelectric crystal form to a paraelectric crystal form at a transition temperature known as a Curie temperature, Tc. Semi-crystalline fluoro-polymers containing the vinylidene fluoride (VDF) group are a particularly relevant example.
It has previously been reported that solid state techniques may be used to produce films from semi-crystalline polymers that have more than one crystal morphology that may be able to inter-convert. However, it has been reported that such techniques are only operable in a narrow temperature range between the crystal-crystal transition temperature on the low temperature end, and the melting temperature on the high temperature end (see “The Strength and Stiffness of Polymers”; Zachariadcs, A. B., Porter, R. S., Eds.; Marcel Dekker: NY, 1983, pp. 1-50.)
Solid-state processing is thought possible in this range because the morphology of the crystallite that is stable above the crystal-crystal transition temperature may have a lower modulus (i.e., stiffness) than the morphology of the crystallite that is stable below the crystal-crystal transition temperature. Often it occurs that the higher modulus crystallite is too stiff to allow solid-state processing below the crystal-crystal transition temperature. (see Aharoni, S. M.; Sibilia, J. P. Polym. Eng. & Sci. 1979, 19, pp. 450-455; and Aharoni, S. M.; Sibilia, J. P. J. Appl. Polym. Sci. 1979, 23, pp. 133-140).
Reports of attempts of solid-state processing of semi-crystalline fluoro-polymers include the following: Peterlin and Elwell (see Peterlin, A.; Elwell, I. H. J. of Mater. Sci. 1967, 2, pp. 1-6) reported reducing the thickness (and expanded the transverse dimensions) by subjecting polyvinylidene fluoride (PVDF) films to a rolling action in the solid state at room temperature. PVDF is a semi-crystalline, VDF-containing polymer that exhibits a crystal-crystal transition at its Curie transition temperature (Tc) where the rhombohedral and hexagonal crystals are at inter-conversion equilibrium. Importantly, in PVDF the Tc is very close to the melting temperature. As known to one of skill in the art, PVDF may be referred to as ferroelectric polymer. PVDF polymer is capable of providing ferroelectric properties at temperatures below its Curie transition temperature. Peterlin and Elwell also reported that rolling the polymers through pinch rollers to compress the polymer substantially increased the orientation of polymer chains in the roll plane.
Nagai et al. (see Nagai, M.; Uehara, H.; Kanamoto, T. Kobushi Ronbunshu (in Japanese) 1996, 53, pp. 555-561; and Nagai, M.; Nakamura, K; Uehara, H.; Kanamoto, T.; Takahashi, Y.; Furukawa, T. J. Polym. Sci.: Part B: Polym. Phys. 1999, 37, pp. 2549-2556) and Nakamura et al. (see Nakamura, K.; Imada, K.; Takayanagi, M. Intern. J. Polymeric Mater. 1972, 2, pp. 71-88; and Nakamura, K.; Nagai, M.; Kanamoto, T.; Takahashi, Y.; Furukawa, T. J. Polym. Sci.: Part B: Polym. Phys. 2001, 39, pp. 1371-1380) reported fabricating chain-aligned PVDF films from PVDF gel films by solid-state, co-extrusion to high draw ratios at 160° C. (i.e., above the PVDF Curie transition temperature and about 10° below the PVDF melt temperature).
The chain orientation and crystallinity of the solid state modified films reported by Nagai et al. and by Nakamura et al. were significantly higher than that of PVDF films made by conventional melt extrusion and stretching processes. The degree of crystallinity, the polar (ferroelectric) crystalline content, and the Young's modulus of the films increased with draw ratio, ultimately reaching a crystalline degree in the range 73-80%, a polar crystal content of 100%, and a Young's modulus in the draw direction of 10.5 GPa at the highest ratios. The ferroelectric and piezoelectric properties were also markedly enhanced and among the highest reported for polar crystal form of PVDF.
Lee and Cakmak (see Lee, J. S. PhD, The University of Akron, 1991; Lee, J. S.; Cakmak, M. Polymer Engineering and Science 1993, 33, pp. 1559-1569; and Lee, J. S.; Cakmak, M. Polymer Engineering and Science 1993, 33, pp. 1570-1582) reported extruded rods of polyvinylidene fluoride trifluoroethylene copolymer p(VDF-TrFE) at temperatures in the range of 120 to 150° C., which is above the Curie transition temperature range (70-115° C.) of p(VDF-TrFE) and below the melt temperature (−150° C.) of these copolymer compositions.
U.S. Pat. Nos. 4,282,277, 4,341,927, 4,363,611, and 4,449,905 are directed to tooling useful for the solid state extrusion of semi-crystalline polymers. The exemplified polymers include polyolefins (e.g., polyethylene, polypropylene, polystyrene, and the like). The co-extrusion temperature ranges recited were between the heat deflection temperature and the crystalline melting temperature of the polymer.
Accordingly, solid state extrusion of polymers having a crystal-crystal transition temperatures has been limited to temperatures in a range between above the crystal-crystal transition temperature, (i.e., above the Curie transition temperature of a ferroelectric polymer) and the melting temperature.
As can be seen, there is a need for a method of solid state formation of ferroelectric polymer films at or below the Curie transition temperature of the polymer.